1. Field of the Invention
The present invention relates to a laser scanning optical device that includes optically anisotropic scanning lenses.
2. Description of Related Art
The mainstream type among image forming apparatuses such as full-color copiers and printers is a tandem system in which four photoreceptors are arranged so as to correspond to the colors Y (yellow), M (magenta), C (cyan), and K (black), and images of the respective colors formed on the photosensitive drums are transferred and synthesized on an intermediate transfer belt. Moreover, such a tandem image forming apparatus is equipped with a laser scanning optical device that uses a deflection means (e.g., polygon mirror) to simultaneously scan four optical beams on their respective photosensitive drums, thereby drawing an image.
Conventionally, there are various approaches proposed to inhibit shading (i.e., uneven light intensities) in laser scanning optical devices. In the approach disclosed in Japanese Patent Laid-Open Publication No. 2001-188188, beams from sources with different shading characteristics are synthesized by a polarizer. As a result, an average light intensity distribution is smoothened, achieving high image quality. Moreover, to correct uneven light intensities, Japanese Patent Laid-Open Publication No. 2005-157325 discloses a light intensity distribution control element with lugs of varying height in a principal scanning direction. By using such a light intensity distribution control element, the difference in uneven light intensities between colors can be decreased.
Incidentally, some laser scanning optical devices are equipped with optically anisotropic (i.e., birefringent) scanning lenses. When such a scanning lens passes an optical beam therethrough, the polarization state of the passed optical beam is changed by birefringence, so that reflectance and transmittance varies along the principal scanning direction of the scanning lens, resulting in an uneven light intensity on a photoreceptor. Particularly in the case of the tandem system, in addition to such an uneven light intensity along the principal scanning direction, there are also differences in uneven light intensities between optical paths for their respective colors, as shown in FIG. 10, causing a problem with reduced quality of an image obtained by synthesizing images of the respective colors.